1. Field of the Invention
The present invention concerns a cathode with an emission layer that thermionically emits electrons given an exposure (stimulation, activation) with a laser beam.
2. Description of the Prior Art
A cathode of the above type is known from DE 10 2005 043 372 A1 as well as from United States Patent Application Publication No. 2007/0064872 A1, for example. The known cathode is a component of an x-ray radiator. The emission layer is formed of a material with a low vapor pressure and a high melting point, such as tungsten. A high voltage with a gradient of up to 20 kV/mm can be applied at a cathode made from tungsten due to the low vapor pressure. Moreover, tungsten exhibits a high reflectivity relative to laser wavelengths, such that a correspondingly high laser power can be injected. Furthermore, tungsten has an optimal heat conductivity (λ), heat capacity (Cp) and density (ρ), such that in the ideal case an electron emission occurs only in the laser focal spot on the cathode. Tungsten is also not susceptible to oxidation and/or contaminations that strongly negatively influence the specification of the cathode.
An x-ray radiator having a cathode with an emission layer that is at least partially roughened and/or porous and/or doped and/or exhibits an intermetallic compound or glass-like carbon (glassy carbon) is described in pending U.S. patent application filed on 23 May 2007 with Ser. No. 11/752,585. The materials used for the emission layer have a satisfactory electron emission only in the case of a relatively large focal area of the laser beam, which leads to a correspondingly severe heating of the cathode.
EP 0 147 009 B1 describes cathodes that are produced from semiconductor materials or other non-metallic solid bodies such as, for example, from bialkalis or trialkalis. Furthermore, metals (for example tungsten and tantalum) are cited in EP 0 147 009 B1 as materials for cathodes.
Moreover, an x-ray tube with a photocathode and an anode is known from WO 98/050056 A1. A photomultiplier is arranged between the photocathode and the anode. A lower optical power is thereby required to generate the x-ray radiation.